The overall goal of this research is to elucidate mechanisms that regulate early development of ovarian follicles. As a first step to approach this question, markers have been identified which are first transcribed in activated primordial follicles in rabbits. Zona pellucida gene transcription occurs initially in oocytes of activated primordial follicles, while expression of FSH receptors and inhibin alpha occurs in granulosa cells during the first stages of morphological differentiation. Thus, expression of these genes can provide qualitative methods to identify activated follicles and characterize early stages of folliculogenesis in vivo and quantitative measures of activation and differentiation in vitro. In addition to identification of morphological changes and granulosa cell proliferation, these markers provide functional definitions of the initial steps in folliculogenesis. Newly established culture methods together with these markers allow experimental testing of putative regulators of: 1) follicle activation and 2) early stages of follicular development as independent molecular events. Preliminary results indicate that intraovarian growth factors are regulators of follicle activation and it is generally accepted that FSH controls follicular development. The studies described in this proposal will test the following hypotheses. 1. Activation of primordial follicles is enhanced by growth factors, resulting in expression of zona pellucida genes by oocytes and ultimately FSH receptor genes by granulosa cells during early stages of follicular development. 2. Expression of FSH receptors facilitates FSH-stimulation of subsequent development of activated follicles, including increases in FSH receptor and inhibin alpha synthesis in granulosa cells. 3. The presence of mature follicles suppresses the number of primordial follicles being activated through paracrine mechanisms. The first two hypotheses will be tested in studies involving in vivo and in vitro manipulation of hormones and growth factors. The third hypothesis will be tested by immunological destruction of mature follicles to evaluate intraovarian feedback mechanisms. Most primordial follicles that are activated will subsequently be lost through atresia. The ability to regulate timing and magnitude of follicle activation could influence the overall reproductive capacity of a given female and ultimately lead to new methods for managing reproductive function in clinical or agricultural settings.